In a thermal printing process, a dye bearing donor web is brought into contact with a dye receiving print media at a print zone. Thermal printing is effected by contacting the donor web with a print head that spans the donor web in a direction transverse to the direction of web travel. To maintain intimate contact between the donor web and receiver during the printing operation, the donor web and print media are partially wrapped around the surface of a print drum. The print drum is commonly driven by a precision stepper motor so that the spacing between adjacent image lines can be precisely controlled. The take-up spool for the donor web is often rotatably driven by a far less expensive DC motor, because its function is simply to accumulate expended donor web. The donor web is supplied by a rotatably mounted supply spool, and a clutching arrangement is used to control the drag on the supply spool to prevent free-wheeling of the supply spool under the influence of the take-up spool motor. Print quality is influenced considerably by tension variations in the donor web during printing. An artifact known as banding can occur when web tension varies. Ideally, the pulling tension exerted by the take-up spool remains uniform throughout the printing cycle. Unfortunately, this ideal is difficult to achieve, especially when relatively inexpensive drive motors are used to effect take-up spool rotation. Also, the diameter of the take-up spool has a variable effect on web tension. As prints are made, the take-up spool diameter gradually increases, thereby altering web tension. Accordingly, it will be appreciated that it would be highly desirable to have a thermal printer wherein print quality is not influenced by the tension exerted by the take-up spool on the donor web.
In commonly assigned application Ser. No. 504,445, there is disclosed a thermal printer in which tension in the donor web downstream of the print zone is reduced to zero during each printing operation. This tensionless condition virtually eliminates the banding artifact and is achieved by rotating the take-up spool at a rate slower than the rate at which the donor web is payed out of the print zone. A two speed motor is used to rotate the take-up spool at two discrete speeds. The first speed is sufficiently slow to produce, during each print cycle, web slack between the print zone and the take-up spool, and the second speed is sufficiently fast to eliminate all web slack between printing cycles. Unfortunately, as the take-up spool accumulates expended donor web, its diameter increases causing the web to be taken up at an ever increasing rate even though the spool rotates at a fixed angular velocity. Accordingly, it will be appreciated that it would be highly desirable to have a thermal printer wherein print quality is not influenced by the diameter of the take-up spool.
Commonly assigned application Ser. No. 542,502 discloses a thermal printer wherein the take-up spool rotates at a speed proportional to the approximate instantaneous diameter of the take-up spool. A signal proportional to the instantaneous diameter is produced by a shaft encoder to monitor the angular velocity of the supply spool from which the donor web is unwound and fed to the print zone. A variable speed motor, operatively coupled to the take-up spool, is responsive to such signal to rotate the spool at a rate equal to or slightly slower than the rate at which expended donor web is paid out of the print zone during the printing operation. Unfortunately, a shaft encoder and other components are required that increase the cost of the printer. Accordingly, it will be appreciated that it would be highly desirable to have a thermal printer that has zero tension on the donor web but does not require components which increase the cost of the printer.
A donor is unwound from the supply spool by the force generated at the nip of the rotating drum and the thermal print head. The peripheral speed of the drum during the print cycle is constant and is equal to the speed of the web during the print cycle. Used donor is typically wound onto the take-up spool by a motor/transmission assembly; but, because the donor supply and take-up spool diameters change as donor is wound and unwound, their respective angular velocities must change correspondingly to maintain equilibrium. These motor/transmission assemblies are designed so that the angular velocity will be greater than or equal to that required to rewind the donor at its smallest take-up spool diameter. As more donor is wound onto the take-up spool, less speed is required. The speed change is usually accomplished with a slip clutch inserted between the motor and the spool. The force added by the slip clutch will add a force component that may compromise the integrity of the dye transfer process, manifesting itself as bands of uneven density across the printed page, commonly referred to as banding. Accordingly, it will be appreciated that it would be highly desirable to have a thermal printing apparatus and method that provides zero tension on the donor web to eliminate banding, but which is simple and inexpensive.